Soil compaction

Soil compaction refers to the formation of dense layers of well packed soil, often at the bottom of the cultivated layer.

The most common causes of soil compaction are tractors, harvesting equipment and implement wheels travelling over moist, loose soils. Soils tend to be more compacted deeper into the soil profile due to the weight of overlaying soil above.

Compaction from tyres is related to

• load
• ground contact pressure (same as inflation pressure)
• wheel slip
• tyre dimensions and construction
• forward speed
• the number of passes (90% of compaction occurs on the first pass).

Tractor power and weight have increased 60 - 80% since the mid-sixties, while the tyre area in contact with the soil has only increased on average 20%. This means that compaction is occurring deeper in the profile, and is therefore becoming more difficult to repair once it occurs.

How does compaction affect soil properties and crop growth?

Compacted soil has smaller soil pores, less pore continuity and greater strength. This makes the soil less suitable for growing crops. The soil has fewer spaces that contain air and water needed for plant growth, is less permeable, can store less soil water and is harder for roots to penetrate. In their virgin state soils can be productive and characterised by excellent physical condition. They allow rapid movement of air and water through the soil, providing an ideal environment for maximum root growth and high yields. However, soils are fragile and easily restructured, especially during cropping. Compaction is an example of this restructuring. If compacted badly enough, a soil does not provide adequate space for root growth and soil animal activity, or allow for rapid movement of air and water. In severe cases, roots are unable to penetrate into deeper soil layers, tending to grow along the top of the pan.

The best way for water and air to move through the soils is in large pores created by decaying plant roots or soil insects and earthworms. External pressures, such as from tyres or tillage tools, cause the large pores to collapse to form smaller pores that are less efficient conductors of air or water. The result is slower water infiltration, poor drainage or poorly aerated soils that limit root growth and nutrient uptake needed for maximum yields. However, some soil consolidation or "firmness" is beneficial. In very loose, uncompacted soils we could not be able to drive over a paddock pulling an implement without becoming bogged. Firming the soil around the seed (also a form of compaction) is necessary for creating good seed-soil contact at planting. This is why press-wheels are important for ensuring even crop emergence. Some crops, like barley, actually seem to prefer strong seed-soil contact and a more densely packed root zone than a very loose one. However, compaction must not be allowed to become excessive and should be minimised in the plant root zone.

What is the ideal structure for soils?

Ideally, soils should be made up of round, irregular shaped aggregates between 5mm and 10mm in diameter. This structure allows for the maximum number of large pores to transport water and air through the soil, and will not restrict root growth. When there are many aggregates less than 5mm diameter (eg. in a fine seedbed), the soil tends to seal off or crust in heavy rain and also slow water movement down through the profile. Soils with fewer large aggregates limit the number of larger conducting pores and restrict the growth of crop roots. When these large aggregates also set hard or are very dense (eg. like the blocks that can arise after cultivating a compacted zone), a large proportion of the soil is not available for the crop to extract water and nutrients.

How do different soil types compact?

Any soil can be compacted, although differing soil types can be compacted to varying extents. Soils high in soil moisture, low in organic matter and with soil particles that vary in size (and so can pack together well) are most at risk of being compacted. Compaction occurs more readily when soils are wet because water acts as a lubricant, allowing soil particles to be easily rearranged. In engineering terms, wet soils have a low shear strength.

Organic matter (including trash) does not compact readily, so if it is mixed with soil particles it limits the amount of compaction that can occur.

A good mix of particle sizes in soil also allows for greater compaction, as the small particles fill the spaces between larger particles that would otherwise be empty if all were the same size. The red soils of the Burnett and Atherton Tablelands were once considered highly resilient to compaction, probably because they once contained very high levels of organic matter. However, compaction can now be found as deep as 700mm into the profile. An added problem with the red soils is their limited ability to "self heal" by swelling and shrinking during successive wetting and drying cycles. This is a characteristic of soils like the black earths on the Darling Downs and Central Queensland. Compaction in red soils is therefore likely to persist for a long time.

What is the difference between compaction and hard setting?

A hard setting soil loses its structure naturally as it dries out – for example, in a surface crust. However, compaction is usually a by-product of another operation such as vehicle traffic during planting. Soils that have a hard setting surface layer are difficult to manage. It is a natural characteristic of some soils, but not red soils. Despite this, there is increasing evidence of hard setting in the red soils of the Burnett and Atherton Tableland regions, and this seems to be associated with a decline in the soil organic matter.

Soil organic matter and microbial activity provide most of the structural bonds required to keep a soil well aggregated. Soil microbes live on crop residues and decomposing roots or litter, so if organic matter is low, microbial activity will also be limited and soil structure (aggregation) will be poor.

What other problems arise from compacted soil?

Erosion: Compaction "blocks" water that is trying to infiltrate the soil, so during storms the rate that the soil can allow the water to infiltrate is less that the rate at which rain falls. In this case, run-off and soil erosion can occur.

Limited crop growth: Compaction can also limit crop growth by restricting crop root access to reserves of soil moisture or nutrients deeper down in the soil profile. Poor establishment is sometimes a direct result of soil compaction.

Machinery wear: In some cases, soil compaction causes excessive wear of machinery points.

How can compaction be detected?

In some years compacted soil will affect yields, while in others it will have no impact. In seasons when there is steady rainfall that occurs throughout the season and the optimum number of plants is established, compaction will tend not to influence yields. In other seasons, compaction can limit yields though reducing the soil’s ability to store soil moisture or to make available stored moisture during extended dry periods. Compaction can actually increase yields in some years, by slowing the rate of crop growth and exploitation of stored soil moisture until follow up rain occurs. This means better use can be made of late season rainfall - particularly if the rain coincides with flowering or grain-fill. However, the situations when this occurs are rare.

In the long-term, if compaction leads to higher rates of soil erosion than would normally occur without compaction, it will eventually decrease the potential yield of a paddock. Similarly, if compaction becomes severe enough to greatly restrict root activity, the number of seasons in which crops will perform well will rapidly decrease and the paddock becomes more drought prone.

The easiest way to check for compaction is to look at the soil. If dense soil breaks into flat plates, usually immediately below the cultivated layer, there is a fair chance the soil has a compaction layer. When the soil is moist, pushing a moisture probe through the profile can sometimes highlight a compaction layer. The probe starts easily, then becomes difficult to push in through the compaction layer, then becomes easier to push in as soil loosens up again deeper in the profile. Other indicators of compaction are standing water (sometimes seen in wheel tracks on the red soils), very high draft requirements for deep tillage and low infiltration even when the soil surface is covered. Soil compaction often shows up as poor plant performance because the soil is not a good growth environment for root growth and function. Common indications are slow plant emergence and thin stands, uneven early growth, abnormal rooting patterns ("right angle disease"), nutrient deficiencies in fertile soil, and stressed plants under dry conditions - despite good subsoil moisture.

A compaction layer is usually measured by calculating the soil bulk density, which refers to the weight of soil in a set volume (grams per cubic centimetre). The higher the bulk density the greater the compaction, with extreme compaction on red soils producing bulk densities of 1.4 g/cc. (This is relatively low compared to some sandier soils, which compact more easily and can reach bulk densities greater than 1.8 g/cc).

How can compaction be avoided?

In an environment where rainfall in unpredictable, it is not possible to completely avoid compaction because there are times when driving on the paddock when it is too wet is unavoidable. This often occurs during a wet harvest. However, compaction can be minimised by restricting the use of high compaction tools such as disc ploughs, header chaser bins and tractors with heavy axle loads. Resist the urge to work the soil too wet by starting a day or two later, and avoid working the soil merely to dry it out. Finally, determine the axle loads and ground pressure of new equipment before you buy it.

Firm soil is beneficial for getting the tractor power to the ground. Tractors are most efficient when they work on concrete, but as concrete is obviously unsuitable for growing crops, a compromise is necessary. One option is to use a series of compacted laneways throughout the paddock. The tractors and harvesting equipment drive on the compacted laneways and the crops grow in between. This practice is commonly referred to as "controlled traffic", and it is gaining popularity in the major cropping areas of Australia. Controlled traffic does not eliminate compaction, but minimises its impact on crop production.

What's the difference between surface compaction and deep compaction?

Surface compaction is usually seen in the tyre wheel tracks. The amount of surface compaction is generally related to the contact pressure in the tyres, although wheel slip can also cause surface compaction. Lowering the contact pressure by increasing the area in contact with the soil surface and reducing wheel slip will reduce surface compaction. Using tracks rather than tyres will also reduce surface compaction.

Deep compaction occurs below the cultivated depth. The extent of deep compaction is related axle load, so the greater the axle load the deeper the compaction extends into the profile.

I already have a compaction problem: what can be done?

Firstly, avoid making it worse by driving on wet soil. Remedial strategies for compacted soils then revolve around tillage practices and/or pastures leys.

Pasture phases: If your soil has poor structure (breaks into large clods when worked), is low in organic matter and has a compaction layer at depth, the only long-term option is a pasture phase. This can be expensive if you can’t use the pasture to harvest seed or run cattle, but after the rejuvenating pasture phase you will have more options to change the methods you grow crops than you have now.

Changed tillage practices: If you have compaction in surface layers, a combination of reduced or zero tillage with controlled traffic is a way of reducing the risk of further compaction while minimising the effect of compaction on crop growth. A deep ripping operation as you set up your traffic laneways may be beneficial - especially if you apply nutrients in the same operation.

Will deep ripping solve a compaction problem?

Deep ripping will help minimise the effects of compaction, but it is not a permanent cure. If you deep rip a paddock and then drive on the paddock again when it is wet – for example, at planting – you are immediately re-establishing the compacted layer. Ninety percent of the compaction occurs on the first pass, so in the wheel tracks you could be back to where you started before the crop comes out of the ground.

One way of removing compaction layers in the soil, at least temporarily, is ripping to about 50mm below the compaction layer using a straight shanked chisel plough or ripper when the soil is dry. If the compaction layer is quite deep, this may require a number of passes to ensure the deeper layers are loosened rather than moved sideways and compressed.

While it can also be difficult to tell how wet the soil is at depth, it is important for the soil to be quite dry. The primary benefit results from the shattering action of the tine being pulled through dry soil. This shattering breaks the big clods into smaller clods but does not completely return the soil to the original good soil structure. Running a ripper in wet soil can be like running a knife through butter, and is at best only recreational tillage. In some cases the smearing caused by tillage in wet soil can recreate compaction at the bottom of the tilled layer.

It is a mistake to assume that just because the soil surface is dry and the tractor is not getting bogged that it is dry enough to deep rip. A vigorous deep-rooted crop or an extended dry period will be required to dry the soil out at depth. In some years it may not be possible to effectively break up a compaction layer by deep ripping. In those seasons it may be better to grow a crop and let the crop roots do some "biological ripping".

How much does compaction cost?

Compaction may only cause a reduction in yield in some seasons. However, in every season compaction increases costs by increasing the energy requirements of soil preparation and the capital cost of the equipment required doing the job. Compaction increases the number of operations required to produce a seedbed and can result in a delayed planting operation, which may be the difference between crop success and failure. It also costs money by reducing rainfall infiltration, thus reducing the water available to the crop and increasing the costs of contour bank maintenance.

Soil compaction can also add to the fuel, oil, repairs and maintenance bill by up to $20/ha. Half the tractor energy required to cultivate the soil is taken up in compacting the soil under the wheels and then re-loosening those same wheel tracks. This means that a tractor of double the size is required to do the same tillage operation as when there was no compaction. This adds substantially to the fixed costs of owning larger tractors.

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